1. Field of the Invention
The present invention relates to an ink-jet printhead and a method for manufacturing the same. More particularly, the present invention relates to a thermally driven, monolithic, ink-jet printhead having a heater that is disposed between dual ink chambers, and a method for manufacturing the same.
2. Description of the Related Art
In general, an ink-jet printhead prints a predetermined image, color or black, by ejecting a small volume ink droplet of a printing ink at a desired position on a recording sheet. Ink-jet printheads are largely classified into two types depending on the ink droplet ejection mechanisms: a thermally driven ink-jet printhead, in which a heat source is employed to form and expand a bubble in ink thereby causing an ink droplet to be ejected, and a piezoelectrically driven ink-jet printhead, in which a piezoelectric crystal bends to exert pressure on ink, thereby causing an ink droplet to be expelled.
An ink droplet ejection mechanism of the thermally driven ink-jet printhead will now be described in detail. When a pulse current flows through a heater formed of a resistive heating material, heat is generated by the heater to rapidly heat ink near the heater to approximately 300° C. Accordingly, the ink boils and bubbles are formed in the ink. The formed bubbles expand and exert pressure on the ink contained within an ink chamber. This causes a droplet of ink to be ejected through a nozzle from the ink chamber.
The thermally driven ink-jet printhead may be further subdivided into top-shooting, side-shooting, and back-shooting types depending on the direction of ink droplet ejection and the direction in which a bubble expands. The top-shooting type refers to a mechanism in which an ink droplet is ejected in a direction that is the same as a direction in which a bubble expands. The back-shooting type is a mechanism in which an ink droplet is ejected in a direction opposite to the direction in which the bubble expands. In the side-shooting type, the direction of ink droplet ejection is perpendicular to the direction in which the bubble expands.
Thermally driven ink-jet printheads need to meet the following conditions. First, a simple manufacturing process, low manufacturing cost, and mass production must be provided. Second, to produce high quality color images, a distance between adjacent nozzles must be as small as possible while still preventing cross-talk between the adjacent nozzles. More specifically, to increase the number of dots per inch (DPI), many nozzles must be arranged within a small area. Third, for high-speed printing, a cycle beginning with ink ejection and ending with ink refill must be as short as possible. That is, the heated ink and heater should cool down quickly to increase an operating frequency. Fourth, heat load exerted on the printhead due to heat generated by the heater must be small, and the printhead must operate stably under a high operating frequency.
FIG. 1A illustrates a partial cross-sectional perspective view of a structure of a conventional thermally driven printhead. FIG. 1B illustrates a cross-sectional view of the printhead of FIG. 1A for explaining a conventional process of ejecting an ink droplet.
Referring to FIGS. 1A and 1B, a conventional thermally driven ink-jet printhead includes a substrate 10, a barrier wall 14 disposed on the substrate 10 for defining an ink chamber 26 filled with ink 29, a heater 12 disposed in the ink chamber 26, and a nozzle plate 18 having a nozzle 16 for ejecting an ink droplet 29′. If a pulse current is supplied to the heater 12, the heater 12 generates heat to form a bubble 28 in the ink 29 within the ink chamber 26. The formed bubble 28 expands to exert pressure on the ink 29 contained within the ink chamber 26, which causes an ink droplet 29′ to be ejected through the nozzle 16. Then, the ink 29 flows from a manifold 22 through an ink channel 24 to refill the ink chamber 26.
The process of manufacturing a conventional top-shooting type ink-jet printhead configured as above involves separately manufacturing the nozzle plate 18 equipped with the nozzle 16 and the substrate 10 having the ink chamber 26 and the ink channel 24 formed thereon and bonding them together. The manufacturing process is complicated and misalignment may occur during the bonding of the nozzle plate 18 and the substrate 10. Furthermore, since the ink chamber 26, the ink channel 24, and the manifold 22 are arranged on the same plane, there is a restriction on increasing the number of nozzles 16 per unit area, i.e., the density of nozzles 16. This restriction makes it difficult to implement a high printing speed, high-resolution ink-jet printhead.
In particular, in the ink-jet printhead having the above-described structure, since the heater 12 contacts an upper surface of the substrate 10, approximately 50% of heat energy generated from the heater 12 is conducted into and absorbed by the substrate 10. Although the heat energy generated from the heater 12 is intended for use in boiling the ink 19 to generate the bubble 28, a significant portion of the heat energy is absorbed into the substrate 10 and only a small portion of the heat energy is actually used in forming the bubble 28. More specifically, the heat energy supplied for the purpose of generating the bubble 28 is consumed, lowering energy efficiency. In addition, the heat energy conducted to other parts of the printhead considerably increases the temperature of the printhead as the print cycles are repeated. Accordingly, since a boiling time and a cooling time of the ink 29 are increased, it is difficult to implement a high operating frequency. Further, several thermal problems may occur in the printhead, making the printhead difficult operate in a stable manner for an extended period of time.
Recently, in an effort to overcome the above problems of the conventional ink-jet printheads, ink-jet printheads having a variety of structures have been proposed. FIG. 2 illustrates an example of a conventional monolithic ink-jet printhead.
Referring to FIG. 2, a hemispherical ink chamber 32 and a manifold 36 are formed on a front surface and a rear surface of a silicon substrate 30, respectively. An ink channel 34 is formed at a bottom of the ink chamber 32 and provides communication between the ink chamber 32 and the manifold 36. A nozzle plate 40, including a plurality of passivation layers 41, 42, and 43 stacked on the substrate 30, is formed integrally with the substrate 30.
The nozzle plate 40 has a nozzle 47 formed at a location corresponding to a central portion of the ink chamber 32. A heater 45 connected to a conductor 46 is disposed around the nozzle 47. A nozzle guide 44 extends along an edge of the nozzle 47 toward a depth direction of the ink chamber 32. Heat generated by the heater 45 is transferred through an insulating layer 41 to ink 48 within the ink chamber 32. The ink 48 then boils to form bubbles 49. The formed bubbles 49 expand to exert pressure on the ink 48 contained within the ink chamber 32, thereby causing an ink droplet 48′ to be ejected through the nozzle 47. Then, the ink 48 flows through the ink channel 34 from the manifold 36 due to surface tension of the ink 48 contacting the air to refill the ink chamber 32.
A conventional monolithic ink-jet printhead configured as above has an advantage in that the silicon substrate 30 is formed integrally with the nozzle plate 40 thereby simplifying the manufacturing process and eliminating the chance of misalignment. Another advantage is that the nozzle 46, the ink chamber 32, the ink channel 34, and the manifold 36 are arranged vertically to increase the density of nozzles 46, as compared with the conventional ink-jet printhead shown in FIG. 1A.
In the conventional monolithic ink-jet printhead shown in FIG. 2, however, since the heater is provided over the ink chamber 32, heat dissipating from the heater 45 upward is initially absorbed in the passivation layers 42 and 43 surrounding the heater 45 while heat dissipating from the heater 45 downward is secondarily conducted into the substrate 30 through the passivation layer 41 and used to generate the bubble 49 by boiling the ink 48 contained in the ink chamber 32.
As described above, there still exist problems of reduced energy efficiency and elevated temperature of the printhead according to repeated printing cycles, complicating implementation of a sufficiently high operating frequency and making it difficult for the printhead to operate in a stable manner for an extended period of time.